1. Field of the Invention
The invention relates to an organic electroluminescent device, and more particularly to a tandem organic electroluminescent device.
2. Description of the Related Art
Recently, with the development and wide application of electronic products, such as mobile phones, PDA, and notebook computers, there has been increasing demand for flat display elements which consume less electric power and occupy less space. Among flat panel displays, organic electroluminescent devices are self-emitting, and highly luminous, with wider viewing angle, faster response, and a simple fabrication process, making them the industry display of choice.
An organic light-emitting diode (OLED) is a light-emitting diode that uses an organic layer as the active layer. In recent years, OLEDs have been gradually employed in flat panel displays. The trend in organic electroluminescent display technology is for higher luminescent efficiency and longer lifetime.
Recently, in order to enhance the brightness of the single pixel and attach full color display, a so-called “tandem type organic electroluminescent device” has been provided. The tandem type organic electroluminescent device comprises a plurality of organic light-emitting diodes which are vertically stacked and cascaded together.
FIG. 1 is a cross section of a conventional tandem type organic electroluminescent device 10. The conventional tandem type organic electroluminescent device 10 comprises a first organic light-emitting diode 20, and a second organic light-emitting diode vertically stacked above the first organic light-emitting diode 20.
Specifically, the first organic light-emitting diode 20 comprises a first bottom electrode 21, a first organic electroluminescent layer 22, and a first top electrode 23, and the second organic light-emitting diode 30 comprises a second bottom electrode 31, a second organic electroluminescent layer 32, and a second top electrode 33. Particularly, in the conventional tandem type organic electroluminescent device 10, the first top electrode 23 is electrically connected and directly contacts the second bottom electrode 31.
Since the resistance of electrode (conductive layer) is lower than that of organic electroluminescent layer, carriers are apt to migrate within the electrodes rather than passing through the boundary energy barriers of the interfaces between electrodes and organic electroluminescent layers, resulting in crosstalk 40 between adjacent pixels, as shown in FIG. 2.
In order to solve the aforementioned problems, a conventional tandem type organic electroluminescent device with very thinner intermediate electrode has been provided, wherein the thinner intermediate electrode provides a sheet resistance about ten to dozens of thousands ohms. However, the electrode is thin enough that reproductively thereof is poor. Another conventional tandem type organic electroluminescent device with patterned intermediate electrode has also been provided to prevent the pixels from crosstalk. The electrode patterning process employing shadow mask is not suitable for use in large display fabrication.
Another tandem type organic electroluminescent device e with layer connecting each OLED device is provided. To enhance efficiency, the connection layer has both electron and hole transport abilities and high optical transmittance.
The connection layer can be a doping organic layer such as N-type doping organic layer, P-type doping organic layer or combinations thereof. However, the performance of tandem type organic electroluminescent device depends on the stability of the connection layer. Since the tandem type organic electroluminescent device utilizes high operating voltage, the carrier transport ability of the connection layer is weakened due to carrier diffusion effect.
US Patent NO. 20030189401 discloses a tandem type organic electroluminescent device 100 having a charge generating layer (CGL) as a connection layer, as shown in FIG. 3.
The tandem type organic electroluminescent device 100 comprises a substrate 110, a bottom electrode 120, a first organic electroluminescent layer 130, a charge generating layer 140, a second organic electroluminescent layer 150, and a top electrode 160. Specifically, the charge generating layer 140 serves as a floating electrode rather than being electrically connected to other electrodes. The tandem type organic electroluminescent device 100 exhibits improved current efficiency and lifetime.
Suitable material of the charge generating layer, however, is currently unavailable and usually toxic (such as V2O5). Therefore, it is difficult to achieve mass production. Moreover, the charge generating layer comprising V2O5 has high resistance, resulting in a high driving voltage of the tandem type organic electroluminescent device.
Therefore, it is necessary to develop a tandem organic electroluminescent device to solve the aforementioned problems.